Restore add card button in the timeline

It will add to the first selected collection, or to the
first one in the menu if no collection selected

app/models/user/filtering.rb

@@ -83,6 +83,11 @@ class User::Filtering
     end
   end
 
+  def single_collection_or_first
+    # Default to the first selected or, when no selection, to the first one
+    filter.collections.first || collections.first
+  end
+
   def cache_key
     ActiveSupport::Cache.expand_cache_key([ user, filter, expanded? ], "user-filtering")
   end

app/views/events/index.html.erb

@@ -5,7 +5,7 @@
 
 <% content_for :header do %>
   <%= render "filters/menu", user_filtering: @user_filtering %>
-  <%= render "filters/menu/add_card_button", user_filtering: @user_filtering %>
+  <%= render "events/index/add_card_button", user_filtering: @user_filtering %>
 
   <h1 class="header__title overflow-ellipsis">
     <div class="overflow-ellipsis">

app/views/events/index/_add_card_button.html.erb

@@ -1,5 +1,5 @@
 <div class="header__actions header__actions--start">
-  <% if collection = user_filtering.single_collection %>
+  <% if collection = user_filtering.single_collection_or_first %>
     <%= button_to collection_cards_path(collection), method: :post, class: "btn btn--link btn--circle-mobile" do %>
       <%= icon_tag "add" %>
       <span>Add a card</span>